Sealed fluidization chamber



Jan. 19, 1965 M. PAHLAvouNl 3,156,335

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INVENTOR. MARCEL PAHLAvouN:

A TTORNEVS YMYM United States Patent lOfi ice Bdd . Patented Jan. 1'9, 1965 SEALED FLUlDlZATlGN CHAMBER Marcel Pahlavouni, Brussels, Belgium, assigner to Solvay & Cie., Sit en Commandite, a simple of Belgium Filed Jan. 13, 19M, Ser. No. 82,426 Claims priority, application Belgium lian. 2l), 195

v 1 Claim. (Cl. 34-57) `be regulated as a function of various factors such asthe transfer cocflicieut,` and in order not to cause aggregation of thematerial. A suitable arrangement for the tubes can be obtained, for example, by installing horizontal and vertical coils and joining their ends to headersra favorable arrangement of the tubes is an arrangement in quincunx, for this increases the heating surface per unit of volume. It is desirable to provide, in the upper part of the fiuidized bed, a bed level above the uppermost level of the tubes, in order to insure that the particles of Wet product introduced do not enter into immediate contact with the tubes; this depth left free varies according to the nature of the product, itsV degreeofwetness or impregna-l tion upon entry into the drier, and the degree of residual wetness or impregnation which can be tolerated inthe product which exits from the drier. On the other hand, the temperature cannot exceed a certain level at any place, for, otherwise, fusionand/or degradation of the polymer would result. The drying of thermoplastics is i particularly difcult when these' are impregnated with inflammable solvent. In this case, special-precautions must be taken in order toavoid iires and explosions. Diverse types of equipment' have already been proposedl for `the drying of solid particles of thermoplastic materials, for" example, vacuum driers utilizing an air Vcurrent,"ve rtical driers using hot air, and rotary driers provided with steam tubes. These latter-'apparatus units are characterized by facility for regulating drying conditions andvby the possibility of continuous operation; however, they-are cumbersome and costly, and, furthermore, their use is not indicated when the drying is accomplished in an atmosphere. of inert gas, for the lack of airtightness of these apparatus units can cause loss of inert gas and of solvent on Vthe one hand and the entry of air-with resulting oxidation of the thermoplastic material and risk of explosion on the otherV hand. i. A.

All these disadvantages can be avoided by effecting the drying in an apparatusutilizing a iiuidized bed. The description ofV an apparatus of -this type, especially adapted for the treatment of plastic materials, hasy already been published (Chemie-Ingnieur-Technik, 1957, `pages 411 and 467), but it is applicableonly to discontinuous drying. The author of that publication lpoints out, among other things, that uncertainty exists as to the possibility of designing `a fluidized-bed drier which functions continuously for the treatment ofmaterials which are sensitive to heat. y

The applicant hasnow found that continuous drying of thermoplastic materials in the form ofV a fluidized ned is nature ofthe material treated and the conditions chosen for drying, but one Vcan estimate it as approximately onetenth of the total depth of the tluidized bed.

It is desirable to maintain the temperature of the bed at a yvalue suiiiciently high that drying is complete andthe gas withdrawn from the fiuidized bed is not saturated.

In order to insure a suilicient supply of heat, it is'necessarythat the temperature of the heating tubes be greater than the temperature of the bed (temperature difference,

lOYto 50 C., forV example). On accountof the agitation in a iluidized bed and the evenness of temperature which results, the temperatureof the tubes can reach a level higherthan-the softening temperature or the degradation temperature of the thermoplastic material so long as the temperature of the bed does not exceed that level.

VEven if the difference between Vthe temperature of the tubes and that of the bed is small, the supply or" heat by the tubes immersed in the bed is important because `the coefficient of `heat transfer is increased and the bundle of tubes presents a large heat exchange surface.

It is also feasible to preheat the gas for iluidization prior to introducing it into the bed, but it is necessary t0 prevent the preheating from raising the temperature `of the gas to a temperature abovethe softening point or the point of degradation ofthe thermoplastic material. The quantity of heat Awhich can Vbev suppliedby the iiuidization gasiis substantially less than that which can be supplied by the tubes. i

The continuous removal of the `dried product is based on the fact that fluidized beds behave Vlike liquids and fol- Y low, among other things, the law oflcommunicating containers. p v

Mln the accompanying drawing, FIGURE 1 represents a sectional elevation oflan apparatus in accordance with the present invention.. f

FIGURE 2 is a plan view, in section, taken along line 2 2, of FIGURE l;

. The fluidization zone, as illustrated in the drawing, is

divided into two or more compartments A and Bwhich communicate with each other, these compartments being deiined by partition 1, which extends downwardly but terminates short of plate 6,thus forming passageway-11 between compartments Aand B; thus the uidized material is forced to pass downwardly through one compartment Y (A), through passageway 1i, to the other compartment (B), and the thoroughness of the drying is appreciably Ven'tirelypossible provided one observes certain conditions andiutilizes appropriate apparatus. Y

The present invention` has as an object a process for being maintained at such a level that the ytemperatureV of fthebed does notgexceedthe softening point or the point of degradation ofthe thermoplastic 'material tombe dried, and the time of drying being determined bythe rate lat' whichone introduces thewet material intothe bed.

The heating tubes canbe heated by means of steam,

hot gas,etc. '.Theyrmustbe separated from each other and `-"from the'walls of theenclosure by'a Asufficient interval to increased. llf desired, these successive compartments can be formed `by several driers placed in series. According to one variant, one can, for example, divide the luidization zone into two compartments, Vthe first, having a larger volume, serving for the dryingV proper, the second,

A `having a smaller volume, serving for finishing and directing the product toward the exit. All of the drying compartments are Ifurnished withrheat exchange tubes 3; optronally, these compartments can be maintained at different temperatures.v Ontheexterior wall 12 of the exit cornpartment B, a little. above the uppermost'level of the tubes maintain. good uidization and, consequently, agood heat v'3,` is positioned a weirgwhich empties into a conduit 9j through which the dried `material is removed after mov- Aing rupwardlythrough lompartment B.' This Weir thus automatically regulates the level-jid ofthe bed andmaintains level 14 below the top of partition 1. l

after this operation, so that Below the fluidization zone, there is a chamber` for distribution of the gas. This distribution zone communicates with the iuidization zone through a perforated plate 6 or other suitable device. If desired, the gas introduced into this chamber (inlet 15) can be preheated. v

Above theuidization zone, and in direct communication therewith, there is provided a disengagement chamber d of suitable height and having, preferably, a cross-sectional areay greater than that of the bed. One can, for

example,'provide this chamber with a cross-sectional area approximately double that of the uidization zone. In the wall of the disengagementchamber, there is, on one side, means for introducing thematerial to be dried, and, on the other side, a conduit for removal of exit gas and vapor.

In order to avoid loss of gas, one can provide means 16 and 1'7,V such as star valves, for example, for rendering the apparatus gas-tight, such means being placed at the inlet for introduction of the product to be dried and in 'the' exit conduit for the dried product. This precaution is especially necessary when one uses an inert gas as iiuid-V izing agent.

Finally, it is useful to adapt the dimensions of the particles to bedried in accordance with the vavailablr-:luidization gas. If necessary, the product to be dried is ltreated in a chopper or other suitable apparatus prior to introduction into the drier. p v

1n order to illustrate the invention without limiting the same, a specic example of operation is given subsequently herein. p

The invention can be applied to the treatment of therf moplastic materials, for example, polyoleiins, polyvinylV chloride, copolymers, etc., regardless of the process by evaporated (water, organic solvent, ormixture).

.',Example i ing chromium oxide and silica and/ or alumina and in the presence of cyclohexane are to be dried. The eiliuent solution of polymer in cyclohexane has a temperature of Nitrogen at 44 C. and containing 0.065 kilogram of water per kilogram is introduced into the gas distribution chamber 5 through inlet 15 at a rate of 18 kilograms of dry gas per hour. This chamber communicates with fluidization zone 2 through the perforations of a perforated plate 6. The temperature of the iiuidized bed under the conditions thus deiined is approximately 81 C.

The withdrawal of dried product through exit 9 corresponds automatically to the continuous introduction of wet polymer at 7 and is regulated by star valve 17. The mean residence time of the particles in the drier is about 2 hours. Their water content at the exit is no more than 0.004 kilogram per kilogram of dried polymer.

The iluidization gas'leaves the apparatus at outlet 10 at a temperature of 78 C. and a water content of 0.161

kilogram per kilogram of -dry gas. The entrainment of polymer fines by the exit gas is of the order of 0.8 percent.

In the case of treatment of low-pressure polyethylene, as in the foregoing example, the temperature of the tubes must be regulated so that the temperature of the bed does not exceed the softening point of the polymer. One thus avoids fusion of the product on the tubes or on other hot points which may exist in the apparatus. The polyethylene of the example has ay softening point of about 125 C. according yto the Vicat method (ASTM No. D-lSZS) c based on a load of 1 kilogram.

Certain other polymers are degraded 'byA heat even before reaching their softening point. This is especially true of polyvinyl chloride, which has a Vicat softening point of the order of 85 to 100 C. under a load of 1 kilogram,

but which, depending upon the conditions maintained in the drier, can be degraded between 70 and 95 C. It is p which said materials are obtained and of the liquid to be Y quite evident that when Vone treats these products the temperature yof the tubes must be such that the temperature of the bed does not reach the degradation point of the polymer.

approximately 150 C. and a pressure of 5 to 6 kilograms Y per square centimeter. Water at C. is added and steam is passed through the mixture to vaporize the cyclohexane and obtain an aqueous slurry of polymer at about 88 C. The polymer, which is lighter, floats and is withdrawn by skimming. The particles have a mean dimen-V sion of two millimeters and contain a very large quantity of water.

The product is passed vinto a Vchopper wherein the particles are reduced to about A0.8 millimeter in diameter, the

-proportion of water presentV being about 0.60 kilogram per kilogram of dry polymer. It is to be noted that the particles of'polymer obtained by precipitation inwater retain proportionately more Water prior to chopping than it is advantageous to separate this liquid water prior to introduction of the polymer into the drier. A Y l As shown in the accompanying drawing, the drying is effected in an apparatus of the type described, partition 1 forming two unequal compartments A andB. `The installation comprises a iiuidization zone 2 (about 2.3 cubic meters) provided with one-inch metallic tubes 3, whose total surface in the bed is about 40 square meters. These tubes are separated one from the other and from the walls by an interval of 'approximately 40 to 45 millimeters. Steam is passed through them at about 2 atmospheres absolute, i.e., they are maintained at about 120 C., the

"steam being used at a rate of 235 kiograms per hour.

The wet polyethylene,r previously chopped in 8, is introduced into 7 and into the disengagement zone 4l of the drier at a rate of 47-2.kilogramsn per hour (controlled by star valve l 16) and passes by gravity into iiuidization zoneZ.

While certain structures,.examples andl process steps have -been described forv purposesof illustration, it is clear that the invention is not limited thereto.

V[I claima Apparatus for continuous drying ofv thermoplastic, particulate solids in the form of a fluidized bed, said apparatuscomprising, in combination:

I v gas-tight means for feedingsolids continuously, at a controlled rate, intothe gagement chamber; a gas outlet in the upper partof said disengagement chamber; ,Y l a generally upright partition in said fluidization chamber, dividing said chamber into-a main drying compartment and a finishing compartment in-open coml munication with each other at their lower ends, said partition being adapted to direct-flow Vof solids from said solids supply means into said main' drying compartment land subsequently into said linishing compartment; y p v solids' withdrawal means in communication with the upper part of said'finishing compartment, said withdraWal means being lprovided with gastight valve means for regulating solids tiow; i y and a Weir positioned between said-finishing compart- Y mentand said, solids withdrawal'means and adapted Vupper part Vof said disen- References Cited in the file of this patent UNITED STATES PATENTS Shaw July 4, 1950 Schleicher Jan. 2, 1951 6 Montgomery Mar. 3, 1953 Zubrzyhi July 2, 1957 Sylvest Sept. 4, 1956 Turneyv Feb. 25, '1958 Krebs July 7, 1959 McCosh Dec. 15, 1959 

